This invention relates to residual gas analyzers, and is more particularly directed to a partial pressure gas analyzer for measuring the partial pressures of gasses in a vacuum process chamber. The invention pertains more specifically to a partial pressure controller for measuring the partial pressures of chamber gasses during a sputtering deposition process.
Numerous types of devices have been proposed for measuring the total pressure or quantity of a gas or gasses in a vessel, yet only a small number of devices have been proposed for sensing the type and partial pressure of each gas within the vessel. The prevalent type of sensor used is a quadrupole-type mass spectrometer, a typical one of which is described in U.S. Pat No. 4,362,936. In these analyzers, the gasses are ionized and then separated in a quadrupole mass analyzer. These analyzers operate on the principle that the ions of various gas species have a unique charge to mass ratio signature. Various other types of mass spectrometers include magnetic-sector and time-of-flight devices. Other sensors, less commonly used in the art, depend on the separation of species by the frequency of oscillation of ions or by the size of the orbit of a gas ion in an applied RF field.
In general, all these types of analyzers depend on ionization and subsequent charge-to-mass filtering, and require relatively high vacuum (10.sup.-5 Torr or better) to achieve good separation. At high pressures, the collisional scattering between ions of the same or different charge-to-mass ratio, as well as collisions induced by neutral atoms, makes the identification of specific species and measurement of their quantity extremely difficult. The conventional approach to solving this problem is to employ a pressure reduction stage that restricts the amount of gas permitted to enter the ionizer, and a separate high vacuum pump that pumps out a large fraction of the gas that enters the ionizer. This additional apparatus adds a time delay to the control of the process gasses, and in addition substantially doubles the cost of measurement. These mass analysis systems are typically burdened with high maintenance costs and require frequent calibration to ensure accuracy.
Other methods of gas analysis, such as infrared line absorption, do not work well at the reduced gas pressures used in a typical vacuum deposition or sputtering process, in which the pressure is typically on the order of 10.sup.-6 atmospheres.
Electron beam excitation has previously been employed only to measure deposition rates of evaporant particles, as discussed in U.S. Pat. No. 4,036,167. For a number of reasons, that structure was inappropriate to analyze background gasses at sputtering pressures.